MICHAEL KASHA 



55 



2100 



2.500 



3000 



2500 



3000 



WAVELENGTH IN ANGSTROMS 

 Fig. 13. Near ultraviolet absorption curves of pyridine at room temperature. Le/t, 

 in isopentane; right, in absolute ethanol, after Kasha and Reid (18). 



sharpening: the tt -> tt* transition is intensified in the solvated pyri- 

 dine. Similar results were obtained by Stephenson (38) . A fuller 

 account of the spectra of this molecule is given in another place (18) . 



B. The Diazines 



The diazines are examples of N-heterocyclics in which the lowest 

 ,1 _^ 77* transition is completely separated from the lowest tt -^ tt* 

 transition. A partial reason (26) is the interaction between the lone- 

 pair orbitals (cf. Fig. 9). Pyridazine (12) has an n -^ it* band at 

 approx. 3500 A (cf. Figs. 12 and 14) which strongly blue-shifts upon 

 changing solvent from hydrocarbon to water. The it ^> tt* absorp- 

 tion at approx. 2500 A in this case undergoes a small blue shift with 

 the same solvent change. 



Pyrazine (12) has an n -> it* band at approx. 3300 A (cf. Fig. 15), 

 which undergoes a marked blue-shift with solvent change from hydro- 

 carbon to water. The tt ^ it* band at approx. 2700 A undergoes a 

 small red shift. The greater relative blurring of the n -> tt* band 

 with this solvent change is characteristic. 



C. Molecules with n -^ tt* Long-Wavelength Shoulders and Tails 



Qiiinoxaline (13) in hexane shows a highly structured n -^ it* ab- 

 sorption region as a long-wavelength shoulder at 3500 - 3800 A, which 



